1. Field of the Invention
The present invention relates to formation of optical patterns by means of a spatial light modulator.
2. Related Background Art
The optical pattern formation (or wavefront shaping) technology of producing a two-dimensional optical pattern at high efficiency of utilization of light (theoretically, at almost 100%) is one of the applications of PAL-SLM (Parallel Aligned Nematic Liquid Crystal Spatial Light Modulator), which is a phase-modulating spatial light modulator. Currently, the optical pattern formation technology is drawing attention in the fields of laser processing and optical tweezers. The optical pattern formation technology can be applied, in the field of laser processing, to laser marking that carves any given pattern, and in the field of particle trapping like the optical tweezers, to multipoint control.
When an optical pattern is formed by modulating the phase of reading light by a PAL-SLM and Fourier-transforming the modulated light, the optical pattern also includes an appearance of 0-order light which is not affected by the phase modulation, in addition to a desired pattern corresponding to the phase modulation. The reason why the 0-order light appears is that there occurs deviation from the ideal phase modulation due to the influence of MTF (Modulation Transfer Function) and the surface precision of the PAL-SLM. Since the 0-order light is noise, it can possibly cause a problem in application fields of optical pattern formation. For example, in a laser processing application, a portion which irradiated with the 0-order light will be strongly processed, and in an optical tweezer application, a sample will be trapped at the position of the 0-order light. Japanese Patent Application Laid-Open No. 2001-272636 discloses a technique of placing a shield plate for the 0-order light on an optical path to block the 0-order light. Japanese Patent Application Laid-Open No. 7-28085 suggests a technique of providing an insensitive portion at a position where the order light is incident in an optically addressing region of the PAL-SLM.